SPC 1730Z Day 2 Outlook
Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 1229 PM CDT Wed Jun 13 2018 Valid 141200Z - 151200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE DAKOTAS... ...SUMMARY... Scattered severe storms are expected across parts of the northern Plains Thursday evening through the overnight. ...Synopsis... A compact, midlevel low, currently moving into southern British Columbia and northwest Washington per water-vapor imagery, is expected to track northeast from northern ID/vicinity of the international border Thursday morning to mainly southern Saskatchewan by Thursday night. A 500-mb ridge axis, initially situated over the Dakotas, will slowly progress east Thursday in response to the northeastward ejection of an attendant mid-level 70-75 kt jet max over eastern Montana, and a deepening upstream large-scale trough across the western U.S. through Thursday night. At the surface late Thursday afternoon/early evening, a cold front will extend south along the ND/MT border and trail westward through northern WY, while a dryline extends south through the central and southern High Plains. Meanwhile, a warm front should be positioned from a triple point located in northwest SD through western IA and northern MO. ...Northern Plains to Upper Midwest... Although much more notable height falls (and related large-scale ascent) are expected across southern Canada, weak height falls are forecast over the western Dakotas mainly from Thursday evening through the overnight period. Meanwhile, a corridor of middle-upper 60s to around 70 F surface dew points will spread underneath a plume of very steep 700-500-mb lapse rates. This combined with diurnal heating will likely promote MLCAPE values of at least 2500-3500 J/kg across parts of the Dakotas by late Thursday afternoon. Initial convective coverage should occur along the cold front as it enters the western Dakotas late Thursday afternoon or by early evening across northwest ND where initial height falls are expected. Otherwise, an increase in low-level convergence near the triple point in northwest SD will be another focus for convective development by Thursday evening. Additional storms will develop as the cold front advances east across the western/central Dakotas, within the residual supply of very strong instability. Ample effective shear will support supercells initially, capable of very large hail and damaging winds. The mesoscale environment for a tornado threat should become more favorable with time and eastward extent, given a nocturnal increase in the low-level jet. However, upscale growth may force less favorable convective modes prior to the environment becoming more conducive for a greater tornado risk. One or more convective systems may organize across North Dakota and northern South Dakota through the early overnight, spreading a hail and damaging wind potential east towards Minnesota through the remainder of the period. Additionally, low-level warm advection may support a few stronger cells towards daybreak Friday with southeast extent across part of the upper Mississippi Valley. However, the position of the upper ridge axis suggests these storms should be sub-severe with some small hail possible. ...MAXIMUM RISK BY HAZARD... Tornado: 5% - Slight Wind: 15% - Slight Hail: 30% SIG - Enhanced ..Peters.. 06/13/2018
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